Higher Biology

DNA Structure


Prokaryotes Vs Eukaryotes


The base sequence of DNA forms the genetic code.


DNA is a molecule made of two strands of repeated units known as DNA nucleotides.


The two strands of DNA twist to form a double helix shape. 


The two strands of DNA are antiparallel and are held together by hydrogen bonds between the complementary base pairs.  



Prokaryotes are organisms whose cells do not contain membrane bound organelles – e.g. bacteria. 


Eukaryotes do contain membrane bound organelles like a nucleus, mitochondria and chloroplast – e.g. animals, plants, fungi (yeast). 



Organisation of DNA in Prokaryotes


As there is no nucleus the DNA is found in the cytoplasm in one of two forms: 


  1. Single circular chromosomes

  2. Plasmids (small rings of DNA). 


You don’t need to know all of the 

parts labelled – but note:

NO NUCLEUS

NO MITOCHONDRIA

NO CHLOROPLAST. 


DNA Nucleotide


Phosphate


DNA


Deoxyribose sugar


Base


DNA Molecule


Organisation of DNA in Eukaryotes


Each end of a DNA strand is known as a prime end – prime is given the symbol ‘. The 5’ end is the end of the strand with the phosphate most exposed. The 3’ end is the end with the deoxyribose sugar most exposed. The strands are said to be antiparallel because one strand runs in the 5’ -> 3’ direction and the other runs in the 3’ -> 5’ direction. 


DNA in Eukaryotes is found in two ways: 


  1. Linear chromosomes within the nucleus

  2. Small circular chromosomes within 

the mitochondria (all eukaryotes) 

and chloroplast (plants only).



YEAST is a special case of a eukaryote as it 

is the only eukaryote that contains plasmids. 




3’


5’


The four DNA bases are adenine, thymine, cytosine and guanine. Adenine and thymine are complementary therefore can only bond with each other, the same is true of cytosine and guanine. 


Formation of Linear Chromosomes


Linear chromosomes in eukaryotes are formed when DNA is tightly wrapped around histone proteins.


U1 KA1


5’


3’


STRUCTURE OF DNA

Unit 1: DNA and the Genome

This unit focuses on the molecular basis of life and evolution

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  • DNA Structure & Organisation: Knowledge of nucleotides, double helix structure, and differences between prokaryotic and eukaryotic genomes

    .

  • DNA Replication: Understanding DNA polymerase, primers, and the Polymerase Chain Reaction (PCR)

    .

  • Gene Expression: Transcription and translation processes, including RNA splicing (introns and exons) and protein folding

    .

  • Cellular Differentiation: The role of stem cells (pluripotent and multipotent) in animals and meristems in plants, including therapeutic and ethical uses

    .

  • The Genome & Mutations: Understanding that most of the genome is non-coding; types of single gene (substitution, insertion, deletion) and chromosome mutations

    .

  • Evolution: Mechanisms of natural selection (stabilising, directional, disruptive), horizontal vs. vertical gene transfer, and speciation (allopatric and sympatric)

    .

  • Genomic Sequencing: Using phylogenetics, molecular clocks, and personal genomics for medicine

    .

Unit 2: Metabolism and Survival

This unit explores how organisms manage energy and survive in varying environments

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  • Metabolic Pathways: Control via enzymes, including competitive, non-competitive, and feedback inhibition

    .

  • Cellular Respiration: Detailed stages including Glycolysis, the Citric Acid Cycle, and the Electron Transport Chain (ATP synthase)

    .

  • Metabolic Rate: Comparing oxygen consumption and circulatory systems (fish, amphibians, reptiles, mammals/birds)

    .

  • Conformers & Regulators: How organisms maintain homeostasis through thermoregulation and negative feedback

    .

  • Adverse Conditions: Strategies like dormancy (hibernation, aestivation) and migration

    .

  • Environmental & Genetic Control: Growing microorganisms in the lab and using recombinant DNA technology (vectors, ligase, restriction endonucleases)

    .

Unit 3: Sustainability and Interdependence

This unit covers global biological issues, from food security to biodiversity

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  • Food Supply & Photosynthesis: Plant productivity and the light-dependent and Calvin cycle stages of photosynthesis

    .

  • Plant & Animal Breeding: Field trials, inbreeding depression, F1 hybrids, and genetic modification

    .

  • Crop Protection: Managing weeds, pests, and diseases through cultural, chemical, and biological controls

    .

  • Animal Welfare: Ethics and behavioral indicators of poor welfare

    .

  • Symbiosis & Social Behaviour: Parasitism, mutualism, and social group behaviors like altruism and kin selection

    .

  • Biodiversity:Measuring genetic, species, and ecosystem diversity, and the impact of habitat loss or invasive species

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